1. Field of the Invention
The present invention relates to an optical reflector for uniformly illuminating a surface to be illuminated in the shape of arc or annulus, an illumination optical system using the optical reflector, a light source system suitable for the illumination optical system, and an illumination optical apparatus provided with the foregoing.
2. Related Background Art
Conventional exposure apparatus for semiconductor production provided with illumination optical system of this type project and replicate a circuit pattern formed on a surface of photo mask (as will be simply referred to as a mask) as an object plane onto a substrate such as a wafer (as will be referred to as a wafer) through a projection optical system. The projection optical system has two reflection mirrors, i.e., a concave mirror and a convex mirror to form a good image in an arcuate region off the axis of projection optical system. Using only the good image region of arc shape, only an arcuate region on mask is projected and replicated onto a wafer. Thus, the replication of the circuit pattern on the entire mask is completed onto the wafer by moving the mask and the wafer in certain directions.
The exposure of this scan type is advantageous in providing a relatively high throughput and a high resolving power.
For exposure apparatus of this type, an illumination optical system desired is one which can uniformly illuminate the whole arcuate region on mask at a constant numerical aperture (NA). Japanese Laid-open Patent Application No. 54-123876 describes a proposal of illumination optical system which can uniformly illuminate a mask in the shape of arc.
The application discloses a belt-like spherical mirror 2 as shown in FIG. 1A, a reflective plane of which has an arcuate shape which is a part of spherical surface 1 cut in the shape of belt. When a light source 4 is placed on a rotation symmetry axis 3 of the belt-like spherical mirror 2, as shown in FIG. 1B and FIG. 1C, a principal ray 5 emitted from the light source 4 is bent at right angle by the belt-like spherical mirror 2 to enter an illuminated surface 6 perpendicularly thereto. Since the reflection is effected on the total reflection plane of arc shape in the belt-like spherical mirror 2, an arc illumination region is formed on the illuminated surface 6.
On one hand, such an illumination optical system has an advantage that the arcuate region can be efficiently illuminated. On the other hand, since the illumination by the illumination optical system is the critical illumination in which a light source image is directly imaged on the illuminated surface, the system has a disadvantage that unevenness of illuminance of the light source 4 appears as illumination unevenness on the illuminated surface 6 as it is. For example, if the light source 4 is an incandescent lamp, a distribution of filament in the incandescent lamp is imaged on the illuminated surface 6 as it is.